International Scholarly Research NetworkISRN Communications and NetworkingVolume 2012, Article ID 132465, 11 pagesdoi:10.5402/2012/132465

Research Article

A Systematic Approach for the Design, Fabrication, and Testingof Microstrip Antennas Using Inkjet Printing Technology

Yahiea Al-Naiemy,1 Taha A. Elwi,2 Haider R. Khaleel,2 and Hussain Al-Rizzo2

1 Department of Applied Science, College of Science and Mathematics, University of Arkansas at Little Rock, Little Rock,AR 72204-1099, USA

2 Department of Systems Engineering, George W. Donaghey College of Engineering and Information Technology,University of Arkansas at Little Rock, Little Rock, AR 72204-1099, USA

Correspondence should be addressed to Yahiea Al-Naiemy, ymainaiemy@ualr.edu

Received 8 February 2012; Accepted 4 April 2012

Academic Editors: C. Luxey, J. Park, H. M. Sun, and K. Teh

Copyright © 2012 Yahiea Al-Naiemy et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

We present a systematic approach for producing microstrip antennas using the state-of-the-art-inkjet printing technique. An initialantenna design based on the conventional square patch geometry is adopted as a benchmark to characterize the entire approach;the procedure then could be generalized to different antenna geometries and feeding techniques. For validation purposes, theantenna is designed and simulated using two different 3D full-wave electromagnetic simulation tools: Ansoft’s High FrequencyStructure Simulator (HFSS), which is based on the Finite Element Method (FEM), and CST Microwave Studio, which is basedon the Finite Integration Technique (FIT). The systematic approach for the fabrication process includes the optimal numberof printed layers, curing temperature, and curing time. These essential parameters need to be optimized to achieve the highestelectrical conductivity, trace continuity, and structural robustness. The antenna is fabricated using Inkjet Printing Technology(IJPT) utilizing Sliver Nanoparticles (SNPs) conductive ink printed by DMP-2800 Dimatix FujiFilm materials printer.

1. Introduction

During the past four decades, microstrip antennas have at-tracted a great deal of attention due to their low profile,ease of fabrication, low cost, and conformability. Inkjet-printed antennas using highly conducting patterns cancomplement and extend the above-mentioned advantages toachieve modern, clean, fast, and reliable antenna fabricationtechnologies. Moreover, the use of nanoscale materials allowsfor the development of a new generation of modern printedcircuit antennas [1–3]. Due to the ever-growing demands forprinted RF circuits and antennas to serve different emergingapplications such as Radio Frequency Identification (RFID),wireless sensors, portable health monitoring, and wearabledevices, several eager attempts from different research groupshave been conducted to investigate the use of conductive inkbased on different nanostructural materials to explore low-cost roll-to-roll production, improve wireless connectivity,structural performance, and flexibility and to reduce the levelof environmental contamination [4–8].

In wireless sensor applications, several RFID designswere reported [9–12] using inkjet technology on low-cost paper substrates. Furthermore, Ultrahigh Frequency(UHF)/RFID printed on flexible, organic, and liquid crystalpolymer substrate have been investigated [13]. In [14],a real-time biomedical monitoring system integrated to awearable RFID-enabled sensor node was reported usingconductive SNP ink printed on low-cost textile substrates. Aperformance comparison of an inverted-F antenna based onnanosilver Inkjet material with respect to identical prototypemade of copper was presented in [9]. Printed bow-tieantennas made of conductive paint were manufactured usingscreen-printing technique and compared to their identicalantennas made of copper [15]. Recently, the integration ofmicrostrip antennas, electronic circuits, and sensors to thepanels of photovoltaic cells is considered as a promisingdesign approach.

The above-mentioned advantages of the inkjet printingin addition to the wide spectrum of applications involved in

2 ISRN Communications and Networking

Antenna printing

Antenna patch curing

Measured conductivity

Antenna parameters measurement

Simulation

p

Figure 1: Design and fabrication processes of the prototypeantenna.

this technique trigger the need to a procedural literature ofthe steps involved in microstrip antennas production.

In this paper, a systematic approach for the design,fabrication, and testing of microstrip patch antennas usinginkjet printing process is presented. An initial antennadesign based on the traditional square patch geometry isinvolved as a benchmark to characterize the methodologyadopted throughout the process. The design and fabricationprocesses are first presented in Section 2. The printer setup,curing process, and conductivity measurement are presentedin Section 3. Section 4 deals with the antenna design andnumerical simulation where two different simulation toolsbased on the Finite Integration Technique (FIT) of CSTMicrowave Studio [16] and the Finite Element Method(FEM) formulation of Ansoft’s High Frequency StructureSimulator (HFSS) [17] are involved to investigate the per-formance of the antenna under consideration. In Section 5,the fabrication process and measurement are introduced;moreover, the printed antenna prototype is compared against

its identical prototype made of copper. Finally, the paper isconcluded in Section 6.

2. Design Process and Fabrication

The systematic approach for the design, fabrication, andtesting of square patch antenna using IJPT is performed infive stages. The complete design and fabrication processes areschematically illustrated in Figure 1.

As shown in Figure 1, there are five main stages in theproposed approach, namely, simulation, antenna printing,antenna curing, conductivity measurement, and antennaparameters measurements. The design and fabrication pro-cesses start with numerical simulation using two differentapproaches: CST MW and HFSS. After the simulationis performed, printing the patch antennas is carried outwith different layers using conductive ink printed usingDMP-2800 Dimatix FujiFilm Materials Printer. The optimalfabrication conditions, such as the number of printedlayers, curing time, feed structure, and complexity of theprinted geometries on different substrates, are discussed.Then, the third stage involves the curing process usinghigh precision convection oven. It is essential to cure theprototype in order to achieve the optimal conductivity of theprinted SNP ink. Using the four-probe technique, the DCelectrical conductivity of the printed SNP patch is measured.For accurate design parameters, the electrical conductivitymeasured value of a printed patch sample from this stage isfed to the first stage as seen in the diagram. The fifth stageinvolves measuring the antenna parameters such as resonantfrequency, S11, gain, and far-field radiation patterns.

Finally the simulated and measured parameters of theSNP prototype are compared.

3. Printer Setup, Curing Process, andConductivity Measurement

3.1. Printer Setup. A Dimatix DMP 2800 Inkjet printer(Fujifilm Dimatix, Inc. Santa Clara, USA) [18] is used toprint the antenna geometries using a disposable piezo Inkjetcartridge that contains 16 nozzles of 23 μm diameter spaced254 μm with typical drop size of 10 pl, drop diameter 27 μm,and capability of jetting 1 pl/10 pl drop.

The DMP printer shown in Figure 2 has 200 mm ×300 mm printing area with a vacuum platen moving in hor-izontal direction integrated with fiducial and drop watchercamera and deposition printer with 10 pl nominal dropvolume cartridge. The vacuumed platen can be heated upto 60◦C. In addition, the wave editor and drop watchercamera allow doctrinaire and control of the electronic pulsesto the piezo jetting device to get optimal drop from thenozzles [18]. A piezoelectrical controller is connected tothe head of the printer through 16 jets. Additionally thefiducial camera allows taking image capture of printedpattern or drops and cartridge cleaning station to clean thecartridge after each single printing process. The antennadesign can be uploaded to the DMP 2800 printer usingDimatix Drop Manager Software in Gerber format file.

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Platen

CartridgeDrop watcher

Print head

Motors

Lid

Fiducialcamera

Figure 2: Photograph of Dimatix DMP 2800 inkjet printer with itsdifferent parts.

Figure 3: Photograph of the ProtoFlow LPKF’s convection oven.

The Gerber file contains all the related geometrical dimen-sions of the antenna design. Furthermore, all printingprocesses and the printing setup conditions can be controlledusing the Dimatix Drop Manager software such as thenumber of the printed layers, heating temperature of theplaten, number of used jets, and height of the cartridge headwith respect to the platen.

Prior to printing the designed antenna, there are severalsteps that should be taken into account. The substrate mustbe cleaned and dried; the substrate holder should be pre-heated to 50◦C to accelerate the evaporation of the solvent;the optimization of droplet spacing in the x and y directionis chosen; finally, the distance between the nozzle tip and the

substrate and the droplet pitch values should be determinedbefore starting the printing process.

3.2. Curing Process. The printed metal patterns need to beannealed to achieve a high conductivity. Therefore, afterprinting the SNP patch, a curing process is utilized in orderto ensure physical continuity and high electrical conductivityof the printed SNP and to prevent oxidization of theprinted surface. The curing process is performed insidethe ProtoFlow LPKF’s high-precision convection industrialoven as shown in Figure 3, which involves two essentialparameters. The annealing temperature and the heatingtime are fixed with initial values at 120◦C and 10 hours,respectively. The conductive ink and printed patterns areheated and burred. A good cure is found after several trials,exposure time control, and different exposure times in orderto cure the conductive ink with maximum annealing andminimal deformation of the substrate.

Exposure times of 5, 10, and 30 minutes do notsufficiently dry the solvent. Different durations are useduntil no deformation of the substrate is observed. Theselected initial temperature and heating time are foundto be sufficient to enable the onset and development ofSNP expansion and grain boundary migration, leading tohomogeneity and continuity in the printed SNP structures.The annealing temperature is fixed at 120◦C since themaximum temperature that the substrates used, Roger, FR4,can endure is in the range of 120◦C to 150◦C.

The maximum heating time reported in the literatureis 10 hours [19]. We have found that when the heatinginterval is increased, the SNPs begin to coalesce and thesize of the grains increases. The curing process is appliedto the printed patches so as to melt the printed SNPs, andgood percolation channels are created for electrons to flowas well as to diminish the created cracks due to nanoscaleglomeration phenomena [20].

Figure 4 depicts the mechanism of SNP sintering.Figure 4 reveals that the SNP grains reach out to the neigh-boring grains to form a continuous printed structure withtime and temperate curing. Therefore, the cracks and poreson the printed SNP are covered up. In addition to cohesiveand adhesive, the printed SNP grains on the substrate areobserved in this figure. The curing must be performedimmediately after the printing to avoid oxidization, whichmay lead to conductivity reduction that degrades the antennaefficiency. Images of the printed SNP before and aftercuring are presented in Figure 4 as obtained from the fieldemission Scanning Electron Microscope SEM (7000F JEOLLtd., Tokyo, Japan) with 15 k accelerating voltage. Accordingto the SEM images in Figure 4, it can be observed that anexcellent flat surface morphology has been achieved afteroptimal curing of printed SNPs.

Five samples are printed on 5 mm × 5 mm patches on aRogers RO 3203 substrate with different heights. Next, thesame five patches are reprinted on FR4 and glass substratesfor comparison with those printed on the Rogers RO 3203substrate. In Figure 5 the SEM images provide top view ofthe printed patches.

4 ISRN Communications and Networking

Incr

easi

ng

con

duct

ivit

y

Before curing room temperature

After curing

(a) (b)

Current +100◦C cured

Figure 4: SEM images of a layer of printed nanosilver ink (a) before (b) after curing.

(m)

(l)

(k) (p)

(q)

(w)

(x)

(b)

(c)

(d)

(h)

(g)

(a)

(n)

(f)

(e) (j) (o) (y)

(i)

Figure 5: SEM images of the printed SNP patches with different layers ranging from one layer to five layers as shown in (a)–(e) before curingand (f)–(j) after curing for the Rogers RO 3203 substrate, FR4 substrate (k)–(o), and glass substrate (p)–(y).

In general, it is found that the patches printed on theglass substrate appear more homogenous but with less degreeof SNP expansion than those printed on Rogers RO 3203and FR4 substrates and they are without cracks as seen inFigure 5.

However, the cracks in the printed patches on Rogers RO3203 and FR4 substrates start to diminish with increasingthe number of layers as depicted in Figures 5(i) and 5(j) for

Rogers RO 3203 substrate and Figures 5(n) and 5(o) for FR4substrate.

3.3. Conductivity Measurement. To start the design usingnumerical simulation, the electrical properties of the mate-rials involved in the antenna design must be fully character-ized. One such important property is the electrical conduc-tivity. The four-probe conductivity measurement technique

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Ammeter

Voltmeter

dddFour point probe heads

SNP composite layer

Current patch

Width

Lengt

h

Height

Figure 6: DC conductivity measurement.

Figure 7: Four-probe (a Keithley 224 programmable current sourceand a Keithley 617 programmable electrometer) devices for meas-uring the electrical conductivity.

SNP patch

h

17 mm

17 mm

Figure 8: Sample of printed conductive SNP with its dimensions.

is a widely accepted approach for measuring the DC electricalresistivity of thin sheet samples [21]. A current sourceinjects stable electrical current with a certain amount passingthough the two outer probes. The voltage drop across the twoinner probes can be measured using a voltmeter as can beseen in Figure 6. The electrical conductivity measurement isperformed using Keithley 224 programmable current sourceand a Keithley 617 programmable electrometer as shown inFigure 7.

The measurement has been conducted using five printedsamples of SNP ink with five different heights. The width

and length of each sample fixed at 1.7 cm × 1.7 cm as shownin Figure 8. For each individual sample, we have consideredtwo voltage readings, in the forward and reverse directions.Table 1 depicts the results of DC electrical conductivity fromone to five layers of SNP patches. Ohm’s law is used tocalculate the DC electrical conductivity for the SNP ink usingformula (1)

ρ = VWh

IL−, (1)

where ρ is the resistivity in ohm·m, V is the voltage measuredby the voltmeter in volts, I is the current measured bythe ammeter in amperes, W is the width of the samplebar measured in meters, h is the height of the sample barmeasured in meters, and L− is the distance between the twopoints where the voltmeter wires make constant to the barmeasured in meters.

The measured DC electrical conductivity for the SNP inkis found to be around an average value of (8.8±0.2)×104 S/m.The small tolerance is due to the difficulty in obtaining twohomogeneous current passages in two directions for the sameSNP sample because of the cracks as can be seen in the SEMimages in Figure 5.

The conductivity of printed SNP is lower than thebulk phase as a result of cracks, pores, surfactants andother impurities [22, 23]. This explains why the measuredconductivity of printed SNP is less than that of bulk silver.The measured value of the electrical conductivity is used inCST MWS and HFSS to numerically evaluate the antennaperformance.

4. Antenna Design and Simulation

As an example, the antenna is chosen to operate at a resonantfrequency of 2.45 GHz since this band is very common andwidely used in the wireless systems.

The transmission line model is used for calculatingthe dimensions of the patch. Three essential parameters,frequency of operation ( fo), height of dielectric substrate

6 ISRN Communications and Networking

Table 1: The DC electrical conductivity measurements.

No. of layers Thickness/nmMeasured

current/mAMeasured voltage/mV

(forward)Measured voltage/mV

(reverse)Conductivity/S/m

1 670 1.00 1.59 1.61 8.77× 104

2 775 1.00 1.42 1.39 8.61× 104

3 817 1.00 1.37 1.25 8.8 × 104

4 1100 1.00 1.02 1.00 8.5 × 104

5 1286 1.00 0.84 0.91 8.55× 104

Patch

Substrate

Ground plane

SMA port

Patch Ground

Wp

YfXf

LpLs

x

x

y

y

z

z

Ws

Figure 9: Geometry of the square patch antenna with the coaxial probe feed and the inset distances from the patch edge.

(a) (b)

(c) (d)

Figure 10: Prototype of the fabricated patch antenna (a) SNP, (b) copper, (c) and (d) the bottom view with SMA connector attached forSNP and copper, respectively.

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0

−2.5

−5

−7.5

−10

−12.5

−15

−17.5

−20

−22.52 2.2 2.4 2.6 2.8 3

Frequency (GHz)

HFSSCST MWSMeasured

S11 (dB)

Figure 11: Comparison of S11 between CST, HFSS, and measure-ment of the square copper antenna.

2 2.2 2.4 2.6 2.8 3

Frequency (GHz)

HFSSCST MWSMeasured

S11 (dB)0

−2

−4

−6

−8

−10

−12

−14

−16

Figure 12: Comparison of S11 between CST, HFSS, and measure-ment of the SNP square patch.

Table 2: Dimensions of the square patch antenna.

Symbol Parameter Dimensions (mm)

Lp Patch length 34.5

Wp Patch width 34.5

Ls Substrate length 60

Ws Substrate width 60

Xf Feed position in x direction 5.5

Yf Feed position in y direction 17.25

(h) and dielectric constant of the substrate, must be selectedappropriately to design a microstrip patch antenna. Thechosen substrate in this research is Rogers RO 3203 of arelative permittivity, εr , of 3.02, loss tangent, tan δ = 0.0016,and thickness of 1.524 mm. Using the equation

εreff = εr+1

2+εr−1

2

[1 +

12hW

](−1/2)

, (2)

Figure 13: Photograph of the radiation patterns measurementinside UALR’s anechoic chamber.

the effective dielectric constant (εreff) is found as 2.826, andby applying the equation

ΔL = 0.412h(εreff + 0.3)((W/h) + 0.264)(εreff − 0.258)((W/h) + 0.8)

, (3)

the extension, ΔL, is found to be 0.746 mm. To calculate theeffective length (Leff), applying equation

(fo)

10 =c

2(L + 2ΔL)√εreff

, (4)

we found Leff to be equal to 36.41 mm. Using (4),

Leff = L + 2ΔL. (5)

The length (L) is found as L = 34.9 mm.The dimensions of the ground plane are determined by

[24]

Lg = 6h + L = 6(1.524) + 34.5 = 43.64 mm,

Wg = 6h + W = 6(1.524) + 34.5 = 43.64 mm.(6)

However, the design of the square microstrip antenna isbased on a larger ground plane (60 mm × 60 mm) to reducethe level of side lobes. The feed point must be located wherethe input impedance is 50 ohms at the resonant frequency.The location of the feeding point is achieved by a trial-and-error method by selecting a distance 30% from the edge andmoving is a small step to (Xf = 5.5 mm, Yf = 17.25 mm),locate the optimum feed point where the S11 is minimum.

In Figure 9, the geometrical details of the square patchare shown.

As illustrated in Figure 9, the dimensions of Lp × Wp

for the microstrip patch and for the substrate, Ls ×Ws, areillustrated in Table 2.

These antenna parameters in addition to the electricalconductivity discussed in the previous section are then fed totwo simulation software based on two different techniques,Finite Integration Technique (FIT) of CST Microwave Studio(CST MWS) and the Finite Element Method (FEM) formula-tion of Ansoft’s High Frequency Structure Simulator (HFSS),which are chosen for validation purposes. In the next section,the simulation results of both softwares are compared to themeasured ones.

8 ISRN Communications and Networking

0

30

60

90

120

150

−180

−150

−120

−90

−60

−30

4

−2

−8

−14

E-plane (dBi)

HFSSCST MWSMeasured

(a)

0

30

60

90

120

150

−180

−150

−120

−90

−60

−304

−2

−8

−14

H-plane (dBi)

HFSSCST MWSMeasured

(b)

Figure 14: Simulated and measured far-field radiation patterns in the (a) E-plane and (b) H-plane for the copper antenna at fo = 2.45 GHz.

0

30

60

90

120

150

−180

−150

−120

−90

−60

−30

−7

−13

−1

−19

E-plane (dBi)

HFSSCST MWSMeasured

(a)

0

30

60

90

120

150

−180

−150

−120

−90

−60

−30

−7

−13

−1

−19

H-plane (dBi)

HFSSCST MWSMeasured

(b)

Figure 15: Simulated and measured far-field radiation patterns in the (a) E-plane and (b) H-plane for the SNP antenna at fo = 2.45 GHz.

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Table 3: Tabulated comparison of the experimental and simulated results.

Copper Sliver

SimulationMeasured

SimulationMeasured

CST HFSS CST HFSS

fo/GHz 2.43 2.39 2.37 2.41 2.39 2.44

S11/dB −22 −17.8 −16.8 −11.9 −11.41 −15.2

Bandwidth/MHz (S11 is −10 dB or lower) 22 28 41 26 63 80

Bandwidth % 9.03 11.6 14.8 10.7 26.3 32.7

fo Shift % 2.4 0.92 — 1.18 2.16 —

Gain/dB 6.3 6.7 5.9 5 5.3 4.6

Theta

z

y

xPhi

(dB

)

5

4

3

2

1

0

−5

−10

−20

−30

−40

−50

(a)

5.5

4

3

2

1

0

−5

−10

20−−30

−40

−50

(dB

)

Theta

z

y

x

Phi

(b)

Figure 16: Radiation patterns of SNP antenna evaluated using (a) CST MWS and (b) HFSS.

5. Fabrication and Measurement

After exporting the Gerber file of the simulated design tothe printer, loading the substrate and adjusting the nozzlesdistances as discussed previously, the printing order is givento print the SNP-based square patch. Next, thermal curing isapplied and feed is applied afterwards. A coaxial feed is usedto excite the microstrip antenna using a microdriller, silverpaste, and soldering station to fix the 50 ohm subminiatureversion A (SMA) port. The SMA is connected to the antennaby carefully drilling the antenna at the specified position.The inner conductor of the coaxial cable is connected andsoldered to the patch, and the outer conductor of the coaxialcable is soldered to the ground plane. The inner conductor ofthe SMA port is connected to the patch through the substrateusing silver paste while the inner conductor of the SMA issoldered to the copper patch. For both patches, the SMA portis soldered to their ground plane.

For comparison purposes, the printed prototype iscompared to an identical prototype made out of copper usinga traditional photolithography technique. The SNP antennaand its identical copper prototype are shown in Figure 10.

The measurement of S11 is performed using an AgilentE5071B Vector Network Analyzer (VNA). The VNA is

calibrated using short −50Ω load-open circuit caliper kit.The test is performed within the 2 GHz to 3 GHz band.The antenna is connected to the VNA through a 50Ωcoaxial cable and set on a foam box to avoid the near-fieldinteractions.

A good agreement between the simulated and measuredS11 has been achieved for copper and SNP prototypes as canbe seen in Figures 11 and 12, respectively. A slight shift inthe resonant frequency has been observed around the designfrequency 2.45 GHz for both the SNP and copper prototypes.The discrepancies between the measured and simulated foand bandwidth are attributed to the unavoidable manufac-turing tolerances, in particular the feed structure.

The measured and simulated results for the SNP andcopper prototypes are listed in Table 3.

The far-field radiation patterns are measured at theresonant frequency in the E-plane and H-plane for the SNPand copper antennas inside the University of Arkansas atLittle Rock (UALR) anechoic chamber, which is shown inFigure 13.

The simulated and measured radiation patterns for theE-plane and H-plane show a good agreement as can be seenin Figures 14 and 15, respectively. The gain evaluated ofthe copper patch antenna from CST MW at 2.43 GHz is

10 ISRN Communications and Networking

6.3 dB, and the gain at the resonant frequency of 2.39 GHz is6.7 dB from HFSS. The measured gain is 5.9 dB at 2.37 GHzas depicted in Figure 14. Furthermore, the gain of the SNPpatch antenna obtained from CST is 5 dB at 2.41 GHz and5.3 dB at 2.39 GHz from HFSS. However, the measured gainis 4.6 dB at 2.44 GHz as shown in Figure 15.

The 3D radiation patterns evaluated from CST MWS andHFSS for the SNP patch antenna are shown in Figure 16.

6. Conclusions

In this paper, a systematic approach for the design, fab-rication, and testing of microstrip antennas using IJPT ispresented. A conventional antenna design based on thesquare patch geometry is adopted in this research as abenchmark, which could be applied to different complexgeometries. The systematic approach for the fabricationprocess includes the optimal number of printed layers, curingtemperature, and curing time, to achieve high electricalconductivity. The antenna is fabricated using DMP-2800Dimatix FujiFilm Material Printer and SNP ink. To conclude,this study provides the antenna community with an easy-to-follow optimized systematic methodology for producingmicrostrip antennas using inkjet printing technology.

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[20] A. Rida, S. Nikolaou, and M. M. Tentzeris, “Broadband UHFRFID/sensor modules for pervasive cognition applications,” inProceedings of the 3rd European Conference on Antennas andPropagation (EuCAP ’09), pp. 2344–2347, March 2009.

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ISRN Communications and Networking 11

[23] M. Mantysalo and P. Mansikkamaki, “An inkjet-deposetedantenna for 2.4 GHz applications,” International Journal ofElectronics and Commnuications, vol. 63, no. 1, pp. 31–35,2009.

[24] C. A. Balanies, Antenna Theory: Analysis & Design, John Wiley& Sons, 1997.

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